DE 195 19 230 [U.S. Pat. No. 6,028,430] describes a monitoring method and a monitoring arrangement for a capacitor bushing for high voltages, the bushing having a tap for a component voltage between its capacitor linings. This known method provides that the component voltage is fed to a detection device that monitors the component voltage for a change, that a change in the component voltage and time information with respect thereto are stored, that the interval between at least two changes is detected and that a fault signal corresponding with the frequency of the component voltage change is generated. This known apparatus comprises a detection device in which the component voltage is monitored for a change, a memory in which a change in the component voltage and time information with respect thereto are filed, a computing element by means of which the interval between at least two changes is detected, and means for generating a fault signal, in which case a fault signal proportional to the frequency of the component voltage change is generated. The tap is led to the detection device by a measurement line via a measurement bushing. The tap delivers a component voltage corresponding with an image of the high voltage present at the interior of the capacitor bushing.
Since the high voltage present at the capacitor bushing has an influence on the measurement signals and can appreciably fluctuate, the measured values detected at the tap can significantly vary. Reliable monitoring of the capacitor bushing is thus not guaranteed.
DE 100 37 432 [U.S. Pat. No. 6,837,432] describes a method and an apparatus for monitoring a capacitor bushing that is acted on by an electrical operating voltage and in which a voltage divider is formed by an electrically conductive lining. In this known method it is provided that at least one measured value of an electrical measurement variable is detected by a measuring tap connected with the lining and by ground potential and is stored, where after detection of the at least one measured value the impedance between the measuring tap and the ground potential is changed and at least one signal value of a measurement signal that then forms is detected by the measuring tap and the ground potential and is stored, where the interval between the instant of detection of the at least one measured value and the instant of detection of the at least one signal value is so dimensioned that possible changes in the operating voltage between the two instants are negligible, where on the basis of the measured value and the signal value a characteristic variable, which is compared with a predetermined target value, is determined by quotient formation and where in the case of deviation of the characteristic variable from the predetermined target value a report signal indicating a fault of the capacitor bushing is formed. In this known apparatus a measuring tap, which is connected with the lining and with a measurement device for detection of an electrical measurement variable, is provided, where the impedance present between the measuring tap and ground potential contains an impedance arrangement with which a switching device is associated. The impedance arrangement has a fixed impedance that is contestable with the measuring tap by a switching device and is separable from the measuring tap. The switching device is connected with a control device. In order to monitor the capacitor bushing the impedance arrangement is initially in a first measuring state in which the switching device is open and the fixed impedance is not connected with the measuring tap. In this first measuring state a measured value of an electrical measurement variable is detected at a first instant and stored in a memory in the sensor. This measurement variable is here the electrical voltage present at the measuring tap relative to ground potential. In this measuring state of the impedance arrangement the impedance is formed by the parallel circuit of the capacitance and the internal resistance of the measuring apparatus. The impedance in this measuring state is termed unchanged impedance. After detection of the measurement variable, the impedance arrangement is transferred to a second measuring state. For that purpose the control device is shifted by the switching device in controlled manner to the closed state. As a result, the fixed impedance is now electrically conductively connected with the measuring tap. The impedance is now formed from the parallel circuit of the capacitance, the internal resistance of the sensor and the fixed impedance. In this second measuring state a signal value of a measurement signal that forms is now detected by the sensor at a second instant and is similarly stored. The measurement signal is the electrical voltage present at the measuring tap relative to ground potential. In this second measuring state the impedance is termed changed impedance.
Since the operating voltage present at the capacitor bushing influences the measurement signals and can strongly fluctuate, the measured values detected at the measuring tap significantly vary. Reliable monitoring of the capacitor bushing thus is not guaranteed.
DE 36 01 934 C2 describes a permanently monitored capacitor bushing arrangement in large-size transformers in three-phase current mains, with three capacitor bushings—each of which consists of a winding body with embedded capacitor linings—each with a measurement terminal connected with the last, outer capacitor lining and with an outer flange capacitance between the last, outer capacitor lining and the earthed flange of each bushing. In this known capacitor bushing arrangement it is provided that the measurement terminals of the three capacitor bushings are each connected by a respective trimming capacitor with an artificial star point settable to ground potential, where a sensor is arranged between the artificial star point and the ground potential. Provision is made for the sensor to be connected with a trigger device that switches off the entire arrangement in the event of change in capacitance of the capacitor linings.
U.S. Pat. No. 4,757,263 A describes determination of capacitance values for monitoring the insulating characteristics of high-voltage bushings.